FIELD OF THE INVENTION
The present invention relates generally to a rate-responsive cardiac pacemaker, and more particularly to a sensor for a rate-responsive pacemaker which is responsive to blood oxygen content, thereby allowing the cardiac rate to closely mimic the natural response pattern of the heart to changing physiological need.
The cardiac pacemaker is perhaps one of the best known electronic marvels of modern medicine, and the implantation of a pacemaker in a patient has become almost a routine operation. The small, electronic device pulses the heart of the patient continuously over an extended period of time, or, in the case of demand pacemakers, monitors the heart's natural operation and provides stimulating pulses only when the heart skips a beat. Pacemakers allow patients with heart problems which would have been either fatal or incapacitating without a pacemaker to resume relatively normal lives.
It will be realized by those skilled int he art that the modern pacemaker is a highly complex device, capable of event sensing, two-way telemetry, and sensing and packing in either ore both of the atrium and the ventricle of the heart. Such pacemakers may be finely tuned by the physician subsequent to implant, and the parameters tweaked to result in optimum pacing performance.
Despite the impressive sophistication of such pacemakers, they represent a compromise due to a single major difference between the healthy heart and a paced heart--namely, the response to activity, exercise, or stress. A healthy heart is create responsive to a number of factors including physical activity or exercise. Variations in the cardiac stroke volume and systemic vascular resistance occur in the cardiovascular system due to physiological stresses such as exercise, temperature changes, postural changes, emotion, hypoglycemia, Valsalva maneuvers, etc.
To maintain adequate perfusion pressure and cardiac output under these stresses, it is necessary to adjust hearth rate. The healthy heart may beat at 60 or fewer beats per minute during response or sleep, and at 120 or more beats per minute during strenuous exercise, for example. The heart paced by a pacemaker which is non-rate responsive will typically beat at a constant rate of approximately 70 beats per minute.
It will be appreciated that the paced heart will supply more blood than is needed during sleep, and may even prevent the patient from sleeping restfully. Even more seriously, patients paced at 70 beats per minute experience substantial difficulty in engaging in strenuous activity. A moderate level of activity such as walking will cause difficulty in some patients. It is apparent that a pacemaker which varies in response to physiological need represents a highly desirable device which will enable a normal active life for patients requiring a pacemaker.
Physiological responsive cardiac pacing must optimize cardiac rate to the level of metabolic need in the absence of normal variable cardiac rate. The simplest answer to this problem is atrial tracking pacing, where the patient has a full or partial AV block and a dual chamber pacemaker pulses the ventricle in response to normal cardiac activity sensed in the atrium. However, this technique is not possible in many patients with sinus bradycardia or atrial fibrillation, and so rate-responsive pacing is necessary to mimic the normal variable cardiac rate.
A variety of physiological responsive pacing systems have been proposed, with the systems using a variety of physiological parameters as the basis for varying cardiac rate. These parameters include blood temperature, various sensed timing signals from the heart, pressure measured within the heart, respiratory rate, nervous system activity, physical activity, and blood chemistry. These systems will be discussed briefly below, and the problems inherent in each of the systems will become evident.
Venous blood temperature is measured in the right ventricle by Cook et al. in U.S. Pat. No. 4,436,092. Since blood temperature has been found to rise during exercise and the corresponding body core temperature increase, blood temperature indicates greater physiological need for blood supply. However, the response of such a system in quite slow. In addition, the system is inexact due to the coarseness at which measurements may be taken, the ingestion of cold liquids, and the effect caused by presence of a fever.
Both the QT interval and the P wave have been used to vary heart rate. The use of the QT interval is discussed in U.S. Pat. No. 4,228,803, to Rickards, and involves detection of the repolarization T wave subsequent to pacemaker stimulation (indicating the Q wave). A shorter QT interval is used to produce a higher paced cardiac rate. This system is slow in response, and not highly specific due to variations caused both by drugs ingested and by the used of pacemaker stimulation rather than using sensed contractions.
The use of the P wave is taught in U.S. Pat. No. 4,313,442, to Knudson et al. By responding to average atrial rate through detection of the P wave, the system varies cardiac rate. This is little more than a dual chamber system, and, as mentioned above, this technique is not possible in many patients with sinus bradycardia or atrial fibrillation. It is also slow due to time averaging, and possibly subject to errors due to faulty signal detection which could drive the heart at a greater that desired rate.
The pressure of blood may be used to determine an appropriate heart rate. Using blood pressure within the heart to regulate heart rate has been the basis for several proposed systems, beginning with the system shown in U.S. Pat. No. 3,358,690, to Cohen. Cohen uses a pressure sensor in the atrium to detect a high pressure condition, and, after a short delay, provides a pacing pulse to the ventricle. This system also assumes that the atrium is operating completely normally, and thus it is not possible to use this system in many patients with sinus bradycardia or atrial fibrillation.
U.S. Pat. No. 3,857,399, to Zacouto, teaches a system that measures either left ventricle pressure or intramyocardial pressure using a sensor located in the left ventricle. This is absolutely unacceptable, since to introduce a sensor through the interventricular septum would be dangerous to say the least. Likewise, a cutdown or percutaneous introduction of such a sensor into the heart through an artery would result in necrosis of the artery.
U.S. Pat. No. 4,566,456, to Koning et al., uses a pressure sensor in the right ventricle, and, in response to either the pressure sensed or the time derivative of pressure sensed, provides a pacing pulse to the right ventricle. This system also assumes that the atrium is operating completely normally, and so it is not possible to use this system in many patients with sinus bradycardia or atrial fibrillation.
Finally, U.S. Pat. No. 4,600,017, to Schroeppel, teaches the use of a pressure sensor in the right ventricle to sense the closing of the tricuspid valve, and provides a pacing pulse thereafter. Once again, if the atrium is not operating completely normally it is not possible to use this system.
A respiratory rate sensor is shown in U.S. Pat. No. 3,593,718, to Krasner. An increase in respiratory rate causes a the system to produce a higher paced cardiac rate. Cardiac rate does not exactly track respiratory rate in the normal heart, and the problem with the Krasner device is that it is either too slow if respiratory rate is time-averaged, or it may be too fast if instantaneous respiratory rate is used. In addition, the system uses variations in chest impedance to produce a signal, making it both subject to false signals due to a variety of causes including loose sensors, and highly subject to damage from defibrillation.
Activities of the central nervous system are highly relevant to modification of cardiac rate. One use of nerve impulses is detailed in U.S. Pat. No. 4,201,219, to Bozal Gonzales, in which a neurodetector device is used to generate electrical signals indicative of nerve impulses. The frequency of the impulses is utilized to modify the paced cardiac rate. The implementation of this is considerably difficult, in that a stable, predictable coupling to the Hering nerve is required. In addition, it is difficult to discriminate between the signals detected to obtain the single signal desired, in that the technology involved is still in its infancy. This approach, while probably having a fast response, thus has neither the sensor reliability nor the system specificity necessary for a reliable product.
The approach which has found its way into the first generation of commercially available pacemakers is the activity sensing variable rate device, which varies rate in response to body movement. As body movement increases, so does the output from the sensor, typically a piezoelectric device producing an electrical output in response to vibratory movement induced by body movement. Increasing output from the sensor causes the system to produce a higher paced cardiac rate. Examples of such devices are illustrated in U.S. Pat. No. 4,140,132, to Dahl, and in U.S. Pat. No. 4,428,378, to Anderson et al.
Activity sensing variable rate pacemakers have a fast response and good sensor reliability. However, they are less than ideal in system specificity. For example, if a person with such a pacemaker was restfully riding in a car on a very bumpy road, his heart rate would increase dramatically at a time when such an increase was not warranted, and, indeed, would not be initiated by the normal healthy heart. Similarly, if the person was pedaling at a furious rate on an exercise bicycle while his upper body were relatively motionless, he would likely run out of oxygen and pass out. Despite the commercial implementation of such devices, it will therefore be appreciated that they are far from perfect.
The last approach which has been taken is to use blood chemistry sensors to detect blood pH or oxygen saturation. The use of pH sensing is taught in U.S. Pat. No. 4,009,721, to Alcidi, and in U.S. Pat. No. 4,252,124, to Mauer et al. A membrane pH sensor electrode is typically placed in the right ventricle, and senses pH, which is proportional to the blood concentration of carbon dioxide, which is generated in increasing amounts by exercise. A diminution in the pH level is used to produce a higher paced cardiac rate. The speed of this system is slow, and sensor reliability over an extended lifetime is not yet great enough to produce a reliable product.
The use of oxygen saturation is shown in U.S. Pat. No. 4, 202,339, to Wirtzfeld et al., in U.S. Pat. No. 4,399,820, to Wirtzfeld et al., in U.S. Pat. No. 4,467,807, to Bornzin, and in U.S. Pat. No. 4,815,469, to Cohen et al. An optical detector is used to measure the mixed venous oxygen saturation, typically in the right ventricle. A diminution in the mixed venous oxygen saturation is used to produce a higher paced cardiac rate. The speed of this system is comparable to the time constant of the body, and sensor reliability and life has been greatly improved to the point where oxygen saturation sensors are fairly reliable devices.
Oxygen saturation systems typically operate using a current source to drive a circuit including the parallel combination of a phototransistor and a resistor in parallel, which combination is connected in series with an LED. The voltage across the circuit is monitored, with the relatively small variation in voltage being indicative of the full scale of oxygen saturation. The voltage across the LED will remain relatively constant, with the amount of current flowing through the resistor determining the voltage across the parallel combination of the phototransistor and the resistor. Despite the resulting variation in voltage across the circuit being small, typically less than 100 mV full scale, it is capable of providing an accurate indication of oxygen saturation.
One of the problems in such circuits is that they are inordinately sensitive to variations in the current source. Typically, a change in the output current from the current source by a given percentage will result in a percentage variation in the voltage range by an order of magnitude. Thus, a one percent change in output current from the current source will result in a variation of at least ten percent of the full scale of voltage variation. The implication of this fact is that the construction of the current source must be highly accurate to maintain even modest accuracy in the measurement of voltage to determine oxygen saturation.
An example of this high degree of sensitivity to a small variation in output current from the current source is helpful in understanding the problem. Assume that the output current from the current source is 1 mA, that the resistor is K.OMEGA., and the full scale variation in voltage across the circuit is 100 mV. Thus, a one percent change in output current from the current source of 0.01 mA will produce a 10 mV change in the voltage across the resistor. This is a ten percent error, and in typical actual circuits the error will be at least ten percent, and typically higher.
In addition, the total voltage across the circuit is typically 3.3 V or more, which means that the pacer must have a voltage doubler in it to produce this voltage. This of course results in increased complexity and power consumption or the driving circuitry. Since the voltage measured is analog, an analog-to-digital converter is also required, making the circuitry required in the pacer even more complex, and further increasing power consumption.
It may therefore be appreciated that there exists a substantial need for an improved oxygen sensor which is both highly accurate and not significantly sensitive to variations occurring in the supply current. Accordingly, it is the primary objective of the present invention to provide an improved oxygen sensor having both the required high degree of accuracy in sensing oxygen saturation and a low level of sensitivity to the occurrence of variations in the level of supply current used to drive the device. It is also a primary objective of the improved oxygen sensor of the present invention that it be capable of being driven by a drive circuit not requiring a voltage doubler, thereby reducing both the complexity and the power consumption of the driving circuitry.
It is a further objective of the present invention that it not require an analog-to-digital converter on the output of the circuit, thereby further reducing both the complexity and the power consumption of the device. The oxygen sensor as used in the implementation of a physiological response variable rate pacemaker must retain the desirable features of fast response, long term reliability, and high specificity. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.